The shale stratum is a hard and brittle stratum with developed micro-pores and high water sensitivity. The shale stratum is extremely subject to the wellbore instability accidents such as well collapse and borehole shrinkage during a process of drilling deep layer large displacement horizontal wells and cluster horizontal wells. Although the oil-based drilling fluid is conducive to maintain stability of well wall and protect the reservoir layer to a maximum extent, thus it is widely used in shale gas horizontal well drilling operation; however, the oil-based drilling fluid causes serious contamination of drill cuttings, which is difficult and costly to be treated. In addition, the supported technologies with respect to construction tools and plugging materials are still immature, which severely limits its large-scale application. The water-based drilling fluid in use has insufficient plugging capacity for heterogeneous micrometer/nanometer pores and cracks in the shale stratum, thus it is difficult to meet the construction requirements on safety and efficiency of the well drilling.
The existing plugging materials have a large particle size and their shape do not match with the cracks, it is difficult to enter into the micrometer/nanometer pores and cracks of the shale stratum, thus the plugging effect is poor.